The earthshine comes from the various parts of the Earth that are turned towards the Moon, and the Sun – all the clouds and oceans and deserts and ice-caps that are illuminated and visible from the Moon contribute to the Earthshine. Which parts contribute most?

We take a representative image of Earth, as seen from space, and investigate where the flux mainly originates.Splitting the above JPG image into the R, G and B channels we can analyses where e.g. 10, 50 and 90 % of the light comes from. That is – we seek the pixels that contribute these fractions of the total flux, and identify them in images. Note that R,G and B refers to other wavelength intervals than the B, V VE1 VE2 and IRCUT bands we have – our B band is bluer than the ‘B’ used in JPG images.

In the three frames we see 3 rows (B, G and R, from the top) – on the left in each panel is the original R,G or B band image, while to the right are the pixels contributing to the 10, 50 and 90 percentiles of the total flux in the image. The order of the panels is: top left 90%, left bottom: 50% and top right is 10%.

In the 90% images at top left we note that the B image (top row) looks different from the R and G images – the light in the B band comes from atmospheric scattering – Rayleigh scattering, and aerosol scattering – as well as the ocean and the clouds; other bands have more of their flux coming from clouds.

Variations in the blue may therefore tell us more about the atmospheric state than do the other, redder, bands. The Rayleigh scattering is due to molecular scattering – as long as the composition of the atmosphere is the same this ought to be constant in time; but some of the blue scattering is also due to aerosols and thus we may have a tool to investigate variations in the aerosol load. The longer-wavelength bands will tell us more about the continents. All bands are quite dominated by clouds – a small cloud can reflect as much light as a larger un-varying continental area.

The above is repeated here on another image of the Earth – more realistic as it is half-Earth. Image from Apollo 8.And here is the B,G,R images and the 90% percentiles:Top to Bottom: B,G,R, Left: R,B or G-band image – right: 90th percentile image.

We again see that the light contributing to the blue image (top) is more diffusely distributed than in e.g. the red (bottom) case where most of the light comes from variable features like clouds. This implies that we should expect larger variability in our albedo data for the red images than the blue image.

Hi Chris, I think there is a lot more scattering in the blue – for good reasons – and that a larger fraction of the blue light will be due to this than is the case for redder wavelengths: Since the scattering is from the clear sky above the clouds we should have a pretty constant amount of scattering – unless we find that a lot of the scattered light comes from altitudes that are at cloud altitudes – something I do not know. Hmmm. SO – ASSUMING that Rayleigh scattering occurs in the atmosphere ABOVE the clouds we should have a larger fraction of constantly scattered light in the blue than in the red. Perhaps the blueness and darkening such of the sky as you climb a mountain tells us something? Oh dear – I think it is that Rayleigh scattering is importnat at cloud altitudes and that most of it does NOT come from above the clouds. Hrmmm.Long wavelengths – hm – in the IR and at mm and radio wavelengths? There is some chance that these long wavelengths reflect from the atmosphere and the clouds and the surface – but we can after all observe at radio wavelengths from the surface of the Earth so some radio gets through. At other wavelengths the atmosphere is opaque to radio which is because of absorption in molecular bands – not scattering.The part we care about is the part that interacts with the irradiation from the Sun – absorbed light at all wavelengths certainly have a climate effect – but proportional to the incoming intensity. The solar irradiance at radio wavelengths is low, compared to visible light. The key idea is that “climate forcing” is a matter of the visible light for all practical purposes. I gotta see about the Rayleigh scattering though!

Hi Peter,this is very interesting indeed. Something that occurs to me is: what variability in albedo might could we expect from this going from red to blue — are we just dealing with increased scatter in blue, which would reduce the signal to noise? If so, are we talking about significant amounts? If atmospheric changes occurred as a result of climate change, could one see this in the different filters. We should bear in mind that half the albedo is missing in the sense that it is at long wavelengths — there might be changes out there too…